1. Field of the Invention
The present invention relates to a rotation control apparatus for controlling the output torque and the rotational speed of a differential actuator having two induction motors and a differential mechanism.
2. Description of the Prior Art
FIG. 1 shows the conventional control circuit for the inverter which drives an induction motor used as an actuator. The control circuit includes an input terminal 101 in connection with an AC power source, an AC-to-DC converter 102 for transforming AC power into DC power, a smoothing circuit 103 for smoothing the transformed DC power, a DC-to-AC inverter 104 for transforming the smoothed DC power into AC power with arbitrary frequency, and an induction motor 105 driven by the AC power supplied by the inverter 104.
The operation of the above-mentioned actuator is as follows. The AC-to-DC converter 102 receives AC power of three-phase, for example, supplied through the AC power source terminal 101 and transforms the AC power into DC power. The smoothing circuit 103 makes the produced DC power smooth. The inverter 104 which operates in a switching manner in a certain sequence transforms the smoothed DC power into AC power with desired frequency, and supplies the AC power to the induction motor 105. The AC power produced by the inverter 104 has the voltage vs. frequency (v/f) characteristics as shown in FIG. 2, and the rotational speed of the induction motor 105 is controlled on the basis of the v/f characteristics shown.
When the inverter 104 produces the output voltage v in proportion to the output frequency f as shown in part of the v/f characteristics of FIG. 2, the induction motor 105 is driven with a constant torque characteristic as shown in FIG. 3(a) and 3(b), or when the inverter 104 produces a constant output voltage v irrespective of variable output frequency f as shown in part of FIG. 2, the induction motor 105 produces a constant output characteristic in a wide speed range.
The actuator torque controller arranged as described above is capable of torque control in the normal operating condition except for the lowest speed range as shown in FIG. 3(a). However, the torque ripple increases and the output is lacking in a sufficient and stable torque in the lowest speed range and therefore the torque control for the induction motor is difficult in this speed range.
When the induction motor 105 is rotated in the lowest speed range, the output frequency of the inverter 104 is under the limit so that the rotational speed control for the actuator is difficult, too.
In order to make up for above-mentioned defect in the lowest speed range, various types of differential actuator controllers using two motors have been proposed as disclosed, for example, in Japanese Patent Publication Laid-Open No. 54-27660 (published in 1979).